We are concerned with characterization of neoplastic urothelium as living cells and tissues in laboratory models. Two tissue culture methods are used. Three dimensional growth is obtained in a matrix of collagen-coated cellulose sponge to study tissue organization. Two dimensional monolayer cultures exposed to a continuous gradient of decreasing oxygen tension in meniscus-gradient culture are used to observe the effects of oxygen gradients on the morphology of cells. For in vivo studies we are using two approaches. Neoplastic urothelium from various heterologous sources is transplanted on the chorioallantoic membrane of embryonated eggs. Ectopic bladders lined by normal urothelium are produced under the skin on the back of experimental rats, and tumor cells implanted into the sacs.